If your life is at immediate risk, there isn't any time for political correctness. It doesn't matter if the adversary is male, female, old, young, human, robot, or gorilla. Shoot he/she/it if you must. The philosophical derivatives of what they're doing and why don't really matter in that moment. All that matter is if you value your life more than the enemy's and if you have the means to preserve your own if so no matter if it costs the adversary's life. It's a situation they've presented to you, at least in this hypothetical.
Some of you seem to have missed that part of the set up of this hypothetical is that the child is going to shoot you and that you're presented with the decision of what to do a moment before that happens (you have the opportunity to shoot first, presumably also with a lethal weapon of some sort though it isn't defined). Waxing on about if the child is really going to shoot or not is to ignore the situation you've been presented with and instead substitute it with an alternate reality you seem to like better, namely one in which you're not forced into making the decision asked of you in the OP and can instead take time to play mind games or do a somersault to safety.